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Chapter 16 Contribution of Source-Sink Theory to Protected Area
Science Andrew Hansen Summary The concept of source-sink population
dynamics may be especially relevant to protected areas. Places set
aside as nature reserves often have steep gradients in climate,
topography, and other abiotic factors that result in spatially
explicit population dynamics occurring within in them. Protected
areas are also frequently placed in relatively extreme parts of the
landscape with regards to climate, soils, elevation, and water.
Consequently, spatially explicit population dynamics may occur
between protected areas and the more moderate surrounding
landscape. The goal of this chapter is to evaluate the contribution
source-sink theory has made to understanding population viability
in and around protected areas. A review of the literature for the
past 20 years indicated that the source-sink concept has been
applied to protected areas primarily in three ways. (1) Protected
areas may be sinks for some species due to the more extreme
biophysical conditions within them. These sink populations may be
vulnerable to loss of source areas in unprotected surrounding
lands. Land use intensification around reserves may drive the
degradation of these sources and reduce viability of the species in
the protected area. (2) The areas surrounding protected areas may
become “attractive” sinks due to human activities and lead to loss
of viability of the source population. Large carnivores appear to
be especially vulnerable to this dynamic. (3) Protected areas may
serve as population source areas that supplement hunted or fished
populations in surrounding areas. Many marine protected areas have
been designated as means of allowing more sustainable fisheries in
surrounding waters. I summarize the conceptual basis of each of
these scenarios, provide examples and draw implications for
conservation and management. Hansen, A. J. 2011. Contribution of
source-sink theory to protected area science. Pgs 339-360 in
J. Liu, V., Hull, A. Morzillo, and J. Wiens, eds. Sources,
Sinks, and Sustainability across Landscapes. Cambridge University
Press.
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Introduction In the 20 years since the publication of Pulliam
(1988) on source-sink population dynamics, the theory has been the
basis for advances in ecological theory (e.g., chapters in this
book). It has also contributed to strategies for conservation of
species. One type of conservation application has involved the
viability of species in and around protected areas. Protected areas
are places where human activities are minimized to provide for the
maintenance of natural ecological processes and native species
(Possingham, 2006). They serve as places for the protection of
nature, research sites into the functioning of intact ecosystems,
benchmarks for understanding change in human-altered systems, and
areas for human enjoyment and renewal (Arcese and Sinclair, 1997).
It is increasingly apparent, however, that many protected areas are
not functioning as envisioned. Ecological processes such as
disturbance regimes have been altered and native species have been
lost in many protected areas (Newmark 1987, 1995, 1996; Rivard et
al., 2000; Parks and Harcourt, 2002). Source-sink theory offers a
basis for understanding some of the factors that influence the
function of protected areas. The spatially explicit population
dynamics that are the basis of source sink theory may be
particularly likely in and around protected areas. Protected areas
may be subject to source sink dynamics more than areas at random
for three reasons. First, protected areas are often portions of
larger ecosystems and native species move out of the protected area
and across the larger ecosystem to obtain needed resources over the
annual cycle (Wright and Thompson, 1935; Craighead,1979; Newmark,
1985; Hansen and DeFries, 2007). Second, protected areas are often
surrounded by areas of more intense land use (Wittemyer et al.,
2008). The resulting gradient in land use intensity and human
interaction with native species can influence birth and death rates
and create source sink dynamics (McKinney, 2002; Hansen et al.,
2005). Hence, protected areas may be sources that maintain
subpopulations in sinks in the surrounding more intensively used
lands. Third, protected areas tend to be located in landscapes with
strong gradients in topography, climate, soils, and other
biophysical factors (Pressey, 1994) that can lead to differential
habitat quality and spatially explicit population dynamics. In
fact, protected areas are typically in the harsher portions of
these biophysical gradients (e.g., high mountains, deserts, low
productivity soils) (Scott et al., 2001) and may have relatively
low habitat quality, resulting in populations moving outside of
protected areas to better habitats seasonally or for some
life-history phases. The lower quality habitats in some protected
areas may result in them being population sinks for some species
(Hansen and DeFries, 2007). Over the past twenty years of
theoretical development, two refinements have been added to the
source sink model that are highly relevant to protected areas.
“Refuge” habitats are places where survival of a population is
relatively high but reproduction is relatively low, so the
subpopulation is a weak source (Naves et al., 2003). This typically
occurs in “suboptimal refuge areas with scarce nutritional
resources but a lower risk of human-induced mortality that may
allow for population persistence”, Naves et al. (2003: 1277). Such
populations are often endangered species that have been displaced
to the edges of their former ranges. “Attractive Sinks” are places
where habitat quality is good allowing potentially high
reproduction and high survival, but where either reproduction or
survival is reduced by forces that are not detected by the organism
(Delibes et al., 2001a). Hence, the organism selects habitat based
on perceived quality, but suffers either high mortality or low
reproduction there due to hazards that are not detected such as
hunting or the presence of pesticides (Gunderson et al., 2001).
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A key issue for protected areas is how the presence of source
sink dynamics influences the viability of the metapopulation
(Figure 1). The original source sink theory focused on how a
subpopulation in a sink could be sustained by dispersal from a
source and maintain a viable metapopulation. Loss of viability from
the source due to loss of individuals to the sink was not expected
because of the assumption that intraspecific density-dependent
competition for the good habitat would lead to the fittest
individuals occupying the source and force subdominant individuals
into the sinks (Pulliam, 1988; Delibes et al., 2001a). This is
highly relevant to protected areas because populations in protected
areas in harsh biophysical settings may be sinks that are
maintained by sources outside of the protected area (Figure 1a).
However, populations in protected areas are vulnerable if land use
change or other factors causes the external source to become a
sink. The assumption of density dependent habitat selection does
not hold, however, in the case of attractive sinks. Because of the
inability of the organism to perceive the threats in the attractive
sink, even the fittest individuals may move from sources to
attractive sinks, reducing population size in the source and
reducing the viability of the metapopulation (Gunderson et al.,
2001; Delibes et al., 2001a, 2001b). Thus, sources or refuges in
protected areas could become inviable if too many individual from
the source are lost to attractive sinks (Figure 1b). A third
scenario may allow sustained harvest of individuals in sink
habitats. Under the density dependent model, protected areas can be
created or maintained as sources which supply surplus individuals
to surrounding areas that are attractive sinks through fishing or
hunting pressure (McCullough, 1996). This scenario is being used as
the basis for creating marine reserves to sustain fisheries in
surrounding areas (Lubchenco et al., 2003). This chapter explores
and illustrates each of these three source sink dynamics with
examples from various protected areas around the world. The
examples were derived from a literature review of studies of source
sink dynamics to protected areas for the period 1988-2008. These
scenarios are graphically illustrates by depicting gradients in
biophysical conditions and thus habitat conditions and land use
intensity relative to protected areas. Following these examples,
implications for management are drawn.
Protected Area as a Sink Because protected areas were often
established in places not productive for agriculture or other
natural resource extraction, they are often in harsher biophysical
conditions than surrounding areas (Scott et al., 2001). These
conditions may constrain organisms directly or reduce primary
productivity and food availability for consumers, resulting in
lower habitat quality in protected areas. If habitat quality is
sufficiently low, the protected area may be a population sink for a
species that is maintained by dispersal from sources in the
surrounding areas that have higher habitat quality (Figure 2). In
this case, the presence of the species in the protected area is
dependent upon the maintenance of the source areas in the
unprotected portion of the landscape (Dias, 1996; Delibes et al.,
2001a). If land use intensification leads to conversion of the
source areas to sinks, the metapopulation is at risk of local
extinction. The implications for protected areas of source sink
dynamics where density dependent habitat selection in the source
regulates movement from sources to sinks were summarized by Delibes
et al. (2001: 283), “The results of the scenario corresponding to
an “avoided” sink [one occupied by overflow individuals from the
preferred source] are well known (Pulliam, 1988).
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The source is extinction resistant but the sink is not. Thus,
reserves must be established in sources since preserving only sink
habitats will probably lead to population extinction.” Example:
Song Bird Population Dynamics in Greater Yellowstone Yellowstone
and Grand Teton National Parks in the Northern Rocky Mountains in
the western United States are centered on the Yellowstone Plateau
and surrounding mountains (Figure 3a). Climate is harsh at these
higher elevations with a snow-free growing season as short as two
months. The volcanically-derived soils are also poor in nutrients
and water-holding capacity. Consequently, net primary productivity
is relatively low in the protected areas (Figure 3b). In contrast,
the valley bottoms along rivers flowing from the Yellowstone
Plateau have substantially longer growing seasons, more fertile
soils, and relatively higher primary productivity (Hansen et al.,
2000).
Hansen et al. (1999) found that bird species were not
distributed randomly relative to these abiotic gradients. Rather,
they were associated with landscape settings that were warmer in
June temperature, lower in elevation, on alluvial parent materials,
and/or with higher primary productivity. These sites were dominated
by the deciduous forest cover types and were relatively rare in the
study area. “Hot spots” where bird species were numerous and
abundant covered less than 3% of the study area and were mostly on
or near private lands (Figure 3c). Only about 7% of the hot spots
were within protected areas. Within the private lands, land use was
more intense near hot spots (Hansen et al., 2002). Rural homes were
placed closer than expected at random to biodiversity hot spots.
The primary means by which rural home development influenced birds
in hot spots was by favoring avian brood parasites and predators
(Hansen and Rotella, 2002). These species were especially abundant
near rural residences. Consequently, those bird species susceptible
to nest parasitism and predation suffered very low rates of
reproduction. In the case of the yellow warbler (Dendroica
petechia), for example, nearly half of nests in hot spots near
homes suffered cowbird parasitism and only about 20% of the nests
successfully fledged young. The American robin (Turdus
migratorius), in contrast, is able to better defend its nests
against cowbirds and predators. Nest success in hot spots near
homes for this species was nearly 50% higher than that for yellow
warbler. An important measure of the consequences of these
differences in reproduction is the net population growth. Estimated
population growth of the American robins was positive in these hot
spots. The yellow warbler population, in contrast, was projected to
be declining in these hot spots, likely due to the increased nest
parasitism and predation associated with rural homes. Projections
of the population dynamics of yellow warblers across the northwest
portion of the Greater Yellowstone Ecosystem revealed that rural
home development near hot spots in the lowlands has likely
increased the probability of extinction of these species even in
protected areas such as Yellowstone National Park (YNP) (Hansen and
Rotella 2002). Due to the harsh climate at higher elevations,
yellow warblers in the hot spots in YNP have relatively low
reproduction and these habitats are likely population sinks (Table
1). In pre-settlement times, populations in these high-elevation
sinks were likely maintained by immigration from the low-elevation
hot spots that were population source areas. Expansion of rural
home development and other intense land uses in the lowlands has
likely converted biodiversity hot spots that were source areas for
species like yellow warbler to population sinks, thereby reducing
the viability of subpopulations in the nature reserves. When the
viability of yellow warbler populations in the
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nature reserves in the study area were modeled assuming no
immigrants from outside the reserves, the probability of extinction
was projected to be high. Nearly half of the simulated populations
went extinct within 50 years. This example illustrates that changes
in biotic interactions in small portions of the private lands can
theoretically lead to increased risk of extinction tens of
kilometers away in nature reserves. Other Examples
Sinclair (1995) suggested that lion (Panthera leo)
subpopulations in woodland habitats outside of Serengeti National
Park in East Africa are sources for subpopulations occupying plains
habitats within the park. Both subpopulations are regulated by
resident prey. Resident prey are relatively stable in the woodland
habitats allowing this habitat to be a net exporter of lions. The
plains habitats are subject to the vagaries of weather and
migratory prey movements, and lions cannot maintain themselves on
the low resident prey numbers. Hence, they are sink habitats. The
metapopulation may not be viable, however, because hunting and
poaching in the source habitats has lead to increased lion
mortality. Careful management of mortality in the source areas is
likely needed to maintain the sink population in the protected
area. While the two examples cited indicate that protected areas
may be sinks dependent upon surrounding sources, it is unclear how
commonly this scenario occurs. These examples may stimulate
additional research in protected areas that have the
characteristics that could lead to this scenario.
Protected Area as Source at Risk from an Attractive Sink
The first scenario represents a conservation concern because the
protected area is a sink for the species and persistence in the
protected area is dependent upon the fate of the source areas in
the unprotected portion of the landscape. In the second scenario
described here, in contrast, the protected area represents a source
habitat. Under the assumption of density dependent habitat
selection in the source, the subpopulation in the protected area is
predicted to remain viable regardless of the fate of the
subpopulation in the adjacent unprotected lands. With the typical
pattern of habitat quality increasing outside the protected area
due to more favorable biophysical conditions (Figure 4), these
surrounding areas are also expected to be sources. Recent
conceptual developments identifying “attractive sinks”, however,
lead to concern that human impacts on the unprotected lands may put
at risk the subpopulation in the source habitats in the protected
area. Gundersen et al. (2001) demonstrated experimentally that high
rates of mortality in a dispersal habitat patch (sink) could lead
to reduced population growth in a source habitat. Delibes et al.
(2001a) used a simulation modeling approach to show that high
enough rates of mortality in sink habitats can lead to extinction
of subpopulations in source areas, depending upon several factors
including demography in sources and sinks, rates of dispersal,
proportion of habitat that is a sink, etc. Delibes et al. (2001b)
provide an explanation for these results that deals with the
ability of organisms to perceive habitat quality. Most source-sink
models assume that animals dispersing actively are able to
recognize and, if possible, avoid substandard (sink) habitats.
Thus, they should first select the source and only poorer
competitors would occupy sinks. However, if individuals fail to
select habitat properly the metapopulation may be at risk. The
subpopulation in the source may become too small to remain viable
if too many individuals disperse to the sink where demographic
performance is poor.
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Attractive sinks are thought to typically arise in association
with human activity. “…the actual anthropogenically caused
poor-habitat conditions may be difficult to detect by individual
animals, as the causes of mortality or reduced breeding would be
different from those in their evolutionary history. Hence
maladaptive behaviors may be expected because most of these
attractive sinks probably will be perceived by animals as
sources.”, Delibes et al. (2001a, Pg 278). These human activities
include hunting and poaching which increase mortality rates and
application of pesticides, which can reduce reproductive rates
(Delibes et al., 2001b). Example: Yellowstone Grizzly Bears When
the grizzly bear (Ursus Arctos) was declared an endangered species
in the United States in 1973, the population in the contiguous 48
states was approximately 250 individuals with most located in
Yellowstone National Park (YNP) (Craighead, 1979). Recovery efforts
lead to a population expansion to the approximately 400-600
individuals today and a range expansion to the public and private
lands surrounding YNP (Schwartz et al. 2006). Extensive demographic
analysis has been conducted on this population by Schwartz et al.
(2007) for the period 1983-2002. Based on reproduction levels, they
suggest that habitat quality is higher outside of YNP, due to
higher primary productivity and the factors that drive it. They
found that the reproductive rate across the population was 0.32
cubs/female/yr, an adequate level to support population growth.
This reproductive rate was found to correlate with population
density and measures of habitat quality (winter severity and
whitebark (Pinus albicaulis) pinecone production). With regards to
mortality, they found that 85% of mortality was caused by humans.
Adult survival rates varied across land allocations. Survival was
highest inside YNP, slightly lower on the federal lands within the
grizzly bear recovery zone surrounding YNP (which has been managed
to minimize bear mortality), and lowest on public and private lands
surrounding the recovery zone (Figure 5). The primary correlates
with survival rates were the density of developments, roads, and
homes, found within a bears home range, plus the amount of time a
bear spent in areas open to ungulate hunting during autumn
(Schwartz et al. in review). Using these demographic estimates to
model population growth across the Greater Yellowstone Ecosystem,
Schwartz et al. (2010) found that YNP and the surrounding federal
lands were population source areas. However, the surrounding
private lands were population sinks (Figure 6). The strength and
size of the source areas are currently sufficient to more than
compensate for the sinks and the population has grown over the
study period. A key question is whether future human development
outside of YNP could lead to mortality rates that result in a loss
of viability of the population in YNP. Simulation models indicate
an increased chance of population decline of the entire population
if average female survival drops below 0.91 (Schwartz et al.,
2007). Currently estimates of female survival (0.95) exceed this
level. However, the human population size, rural home density, and
backcountry recreation are growing rapidly in GYE (Gude et al.,
2007). Regulatory agencies have concluded that future risks are
unlikely to be sufficient to cause population extinction and the
species was delisted in the U.S. in April 2007. The potential for
loss of habitat quality under climate change (e.g., loss of the key
food species, whitebark pine) and increasing human-caused mortality
associated with human population expansion has lead conservation
organizations to challenge this delisting of the species. In sum,
these studies suggest that the private lands surrounding YNP
represent an attractive sink for Yellowstone grizzly bears. This
sink currently does not appear to put the
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metapopulation at risk. However, careful monitoring of human
development and bear demography across the system is clearly
warranted to anticipate and prevent loss of viability of the bear
source areas in Yellowstone National Park. As part of the delisting
process, the Conservation Strategy (USFWS 2007) restricts all forms
of development within Yellowstone National Park and the Recovery
Zone to 1998 levels, requires state and federal agencies to monitor
grizzly bear demographics, and document changes developments to
ensure continued viability of the bear source areas within
Yellowstone National Park and the Recovery Zone Other Examples
Brown bears (Ursus arctos) in the Cantabrian Mountains of Spain are
an endangered relict from a distribution that once covered the
entire Iberian Peninsula. Human induced mortality is thought to be
the primary driver of the range constriction. Naves et al. (2003)
introduced the concept of population refuges to describe weak
source areas for brown bears where reproduction is low due to poor
habitat and mortality is low due to little human influence. They
found that the brown bear population in the eastern portion of the
study area mainly occupies areas of suboptimal natural habitat and
relatively low human impact. Bear demographic data suggest that 38%
of this area may function as a source and 41% as a refuge due to
low habitat quality but also low human influence. The western
population was located mainly in areas with high human impact but
otherwise good natural quality. Some 41% was considered attractive
sink and 16% as sink. Both reproduction and mortality rates were
higher in the western population and it experienced a mean annual
decrease of 4-5% during 1982-1995. Although this example does not
involve protected areas, it does illustrate how habitat quality and
human activity often are correlated with humans being concentrated
in the highest quality habitats and that this can have large
effects on population viability. The authors concluded that the
main management goal in the western population should be the
reduction of human-induced mortality and, in the eastern
population, extensive reforestation to increase habitat quality.
The pattern of elevated mortality for large carnivores outside of
protected areas was found for several mammals in subsaharan Africa
(Woodroffe and Ginsberg, 1998), lynx (Lynx pardinus) in Spain
(Gaona et al., 1998), grizzly bears in the US and Canadian Rockies
(Carroll et al., 2004, Neilsen et al., 2006), and wolves (Canis
lupus) in the midwest US (Haight et al., 1998). Lynx in the Iberian
Peninsula show a similar demography across habitat quality and land
use gradients to brown bears studied by Naves et al., 2003). there.
Gaona et al. (1998) concluded that Donana National Park was a
likely population source and surrounding areas a likely sink due to
human-induced mortality. The estimated probability of extinction of
the population within 100 years is 22-34% depending on assumptions.
In the Rocky Mountains of the US and Canada, Carroll et al.(2004)
simulated spatially-explicit demography of grizzly bears and
concluded that increasing the sizes and connectivity of protected
areas increased population persistence.
The examples above suggest that it is not uncommon for some
wildlife species to roam to what appear to be high quality habitats
outside of protected areas and suffer high levels of human-induced
mortality. Species with particular life history strategies are
likely most susceptible to this situation. Specifically, those more
likely to come into contact with humans, such as species with large
home ranges that extend outside protected areas (Woodroffe and
Ginsberg, 1998) or large predators that are dangerous to livestock
that are less likely to be tolerated by people surrounding
protected areas. Also, species likely to have strong
demographic
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response to human induced mortality, such as those that are
long-lived, have high survival rates, and exhibit population growth
primarily set by female mortality (Naves et al., 2003). I am
unaware, however, of any documented cases of mortality in
attractive sinks outside of protected areas being sufficient to
cause the extinction of a source population in the protected area.
The high rates of extinction of large carnivores in many African
protected areas (Woodroofe and Ginsberg, 1988) are consistent with
this hypothesis. Perhaps, development of the concept of attractive
sinks will spur additional research for potential vulnerable
species in protected areas.
Protected Area as Source Allowing Sustainable Harvest in
Surrounding Areas
The third scenario is a case where population sources in
protected areas are used to support human harvesting in the
surroundings. Under this model, habitat quality is sufficiently
high in the protected area for it to be a population source area
(Figure 7). No assumptions need be made about habitat quality
outside the protected area. Individuals dispersing outside of the
protected area are harvested, creating a sink. However, the level
of harvest is controlled to maintain the protected area as a
source. This approach has been advocated to support hunting systems
surrounding terrestrial protected areas and for the creation of
marine protected areas to allow a more sustainable fishery than
traditional methods. Detailed examples of the results of this
approach are few and many questions remain. Terrestrial
Applications The concept that the productivity of game in hunted
areas can be supplemented by movement from source areas has
received widespread attention only in recent decades. McCullough
(1996) examined harvest theory in the context of
spatially-structured populations. He described a harvest system
where the landscape is divided into patches with a minimum patch
size large enough to contain a viable subpopulation of the target
species. A subset of these patches is withdrawn from harvest in
order to ensure a viable population and to act as source areas.
Harvest on the remaining patches may remove all or some of the
population within them. Future harvest will then be dependent upon
either dispersal from the protected patches or additional
production within the hunted patches. The proportion of the
landscape open to harvest may be increased up to the point where
total harvest is maximized, which will be the maximum sustainable
yield. By strictly enforcing no hunting in an adequate proportion
of source areas, this system is protected from the accelerating
overharvest that is characteristic of quota-harvest systems
(McCullough (1996). A similar approach was presented by Joshi and
Gadgil (1991). Novaro et al. (2000) examined application of spatial
control of harvest in the Neotropics. The previous nonspatial
approaches for sustainable hunting that had been applied in the
Neotropics had largely ignored the potential for dispersal into
hunting areas and assumed that animal production was derived from
within the hunted patch. Novaro et al. (2000) suggested that
dispersal into hunted areas from surrounding areas can be
significant. They reviewed the literature on subsistence hunting in
the Neotropics and found that hunting is often conducted in areas
adjacent to relatively undisturbed habitat that may act as sources
of animals for the hunted sites. They also identified species that
may show such dispersal dynamics. Many of the potential sources for
game populations they identified occur in protected areas. They
conclude
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that recognition of protected areas as vital sources of game is
important in that it would help change the negative regard which
many local people have for these areas. Strong empirical tests of
the role of protected areas in supporting source-sink hunting
systems have apparently not yet been done in terrestrial systems.
Navaro et al. (2005) used empirical data and simulation models of
culpeo foxes (Pseudalopex culpaeus) in a study area in Argentina
and concluded that they exhibit source sink dynamics between cattle
ranches (no hunting) and sheep ranches (hunted). Circumstantial
evidence was provided by Naranjo and Bodmer (2007) indicating the
Lacandon Forest reserve in Mexico act as a source for ungulates
hunted on surrounding lands. Source-sink theory was used as the
basis for a management plan for sustainable harvest of puma (Puma
concolor) in the western US based on protecting animals in source
areas. Laundre and Clarke (2003) concluded that closing 63% of puma
habitat to hunting would ensure long-term puma population viability
while permitting traditional hunting levels in other areas. Marine
Applications Source-sink theory also stimulated considerable
thinking on the benefits of marine protected areas (MPA) for
sustainable fisheries. Crowder et al. (2000) explained that the use
of MPA is attractive because they can provide insurance against
management uncertainty and can simplify the management of
fisheries. The latter is true because an area completely closed to
fishing is easier to police than one in which multiple gear
regulations must be enforced. Like terrestrial protected areas,
emigration of fish from sources to attractive sinks is expected to
be driven by density-dependent habitat selection. An additional
mechanism of dispersal in marine systems is water-borne larval
export. Eggs and larvae produced in larger numbers by the greater
spawning-stock biomass within reserves are transported to habitat
outside of the protected area by water currents, leading to an
increased recruitment in the fished areas. However, these
mechanisms lead to substantial challenges in developing source-sink
fisheries. Defining habitat quality and positioning MPA in
population source areas are difficult in oceanic ecosystems
(Crowder et al. 2000). The extreme complexity and unpredictability
of marine currents makes it very difficult to backtrack dispersed
larvae to their physical sources. Thus, designing the spatial
distribution of MPA and harvest areas is very challenging. The most
widely cited study demonstrating a sustained yield fishery using
MPA is Russ et al. (2004). From 1983-2001, they monitored changes
in biomass of surgeonfish (Acanthuridae) and jacks (Carangidae) in
a no-take reserve occupying 10% of the coral- reef fishing area at
Apo Island, Philippines, and at a site open to fishing. Underwater
visual censuses were used to assess fish densities in and outside
of the MPA. They found that: fish biomass in the MPA increased
three-fold during the study period; fish biomass did not change
significantly across the entire harvest areas, however density did
increase in proximity to the MPA; and fishery catch in the harvest
zone was higher during 1985-2001 than before the MPA was
established. The authors concluded, “The benefits of the reserve to
local fisheries at the island were higher catch, increased catch
rate, and a reduction in fishing effort. The fishery and tourism
benefits generated by the reserve have enhanced the living standard
of the fishing community.” (Pg 597). In reviewing the state of MPA
as a basis for sustainable fisheries, Sale et al. (2005) concluded
that the approach is widely advocated. However, rigorous empirical
tests of the effect of MPA designation on fish density within the
reserves are few, and well studied effects outside
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of reserve borders are even rarer. The current network of MPA is
small and often not placed on population source areas due to
socioeconomic factors and lack of knowledge. Gaps in knowledge
include:
Distance and direction of dispersal, especially of marine
larvae, but also juvenile and adult phases, which are needed to
determine MPA size and location;
Water movements near coastlines, and their effects on dispersal,
and on siting of MPA;
Spatial patterns of habitat quality and population performance;
The effects of release from fishing on ecosystem processes such as
trophic cascades
which may inhibit recovery of target species density in MPA;
Well studied cases of MPA establishment leading to a sustained
yield fishery.
Acknowledging and remedying these gaps in knowledge should lay
the basis for more effective application of no-take fishery
reserves. In sum, the use of terrestrial and marine protected areas
as sources for sustainable harvest in the surrounding lands is
supported by theory and has widespread appeal. Successful
application of the approach will require greater knowledge on
spatial patterns of habitat quality, target species demography and
movement, ecosystem processes, and landscape design to achieve
sustainable harvests. This use of protected areas provides
additional justification for the creation and maintenance of
protected areas.
Conclusions and Management Implications During the two decades
since the publication of Pulliam (1988), source-sink theory has
strongly influenced theory and practice involving conservation in
and around protected areas. The available studies on this topic
largely depict three categories of applications: protected areas as
sinks dependent upon outside sources, protected areas as sources
vulnerable to extinction due to human activity creating attractive
sinks in the surroundings, and protected areas as sources
supporting sustainable harvest in surrounding areas. Current
knowledge on these relationships points to various management
strategies to increase species viability within protected areas and
to increase their contribution to sustainable harvest in
surrounding areas. Defining greater ecosystems. Connectivity of
subpopulations within protected areas with those outside is one
example of the ecological processes and flows that often link
protected areas to some larger surrounding ecosystem (Hansen and
DeFries, 2007). Defining such “greater ecosystems” (Grumbine, 1990)
is essential for developing a management approach that places the
protected area in the context of the surrounding area that
influences its functioning. Criteria for defining such greater
ecosystems are presented in Jones et al. (2009) and DeFries et al.
(2009). Among these criteria are population source and sink areas.
Create or expand protected areas. Existing protected areas were
often established without the benefit of knowledge of source-sink
relationships or other spatially-explicit processes. Identifying
and maintaining source areas that are outside protected area
boundaries may be necessary to ensure population viability within
protected areas. Innovative approaches for functionally expanding
the sizes of protected areas such as use of conservation easements
are being widely employed in many places globally (Theobald et al.,
2005). Newly created reserves
-
can be placed and designed based on source-sink and other
current theory to better maintain natural ecosystem and possibly
support sustainable harvest in surrounding landscapes or seascapes
(Margules and Pressey, 2000). Modify human behavior in attractive
sinks. In many cases, the negative effects of human activities on
the demographics of at risk species are unintended and/or
avoidable. Examples of unintended impacts include road kill, death
or displacement of wildlife by pets, and spread of disease by
livestock or by feeding stations. Intended but avoidable impacts
include killing of dangerous animals to protect human lives or
property. Both unintended and avoidable impacts can often be
greatly reduced through education of local residents. Effective
strategies are often low in cost and effort and may be widely
embraced and supported by local citizens who appreciate the
benefits of living more sustainably (Rosenzweig, 2003: Hansen et
al., 2005). Monitor population demography. The functioning of
source-sink systems may be very sensitive to the proportions of
source and sink habitats, population vital rates, lags in
population response, and many other factors (Doak, 1995; Delibes et
al., 2001b; Gunderson et al., 2007). Incomplete demographic
information may be misleading on the status of populations in
sources and sinks. For example, animal censuses in places that
function as attractive sinks could lead to misinterpretation as the
apparently stable or growing sink population could be misidentified
as a source (Delibes et al., 2001a). Furthermore, harvest or other
human induced mortality in the sink could be temporarily sustained
by the sources, before resulting in a rapid collapse of the whole
system. Careful demographic monitoring is essential to generate
knowledge of management strategies that will maintain the function
of source-sink systems. Progress is being made on diagnostic tools
in this regard. Jonzen et al. (2005) offer a statistical method for
estimating monitoring efforts in source or sink habitats needed to
detect declining reproductive success in source habitats. This
review leads to the conclusion that much research, monitoring, and
creative management is yet required to sustain source-sink
populations in and around protected areas. However, the source-sink
concept has thus far greatly enriched conservation biology and has
considerable promise for future contributions.
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Figure Legends Figure 1. Types of source and sink populations
based on levels of survival and reproduction. Subpopulations in
protected areas are denoted by the blue shading and those outside
of protected areas in gray shading. Interactions between sources or
sinks in protected areas and those outside of protected areas are
indicated by the red lines. (a) A sink subpopulation in the
protected area is maintained by a viable source subpopulation
outside the protected area. (b) A source or refuge subpopulation in
a protected area loses viability due to emigration to an attractive
sink. (c) A source subpopulation in a protected area provides for a
sustainable harvest in attractive sinks in surrounding areas.
Figure 2. Model depicting subpopulations in a protected area being
a sink maintained by an external source under natural conditions
(top). This results from the protected area being placed at the
harsher end of the habitat quality gradient across the landscape
(in the absence of human impacts). Human presence in the landscape
leads to conversion of the unprotected source to a sink, reducing
the viability of the subpopulation in the protected area (bottom).
Figure 3. (a) Shaded relief map of the Greater Yellowstone
Ecosystem (GYE) in the Northwestern United States. Black polygons
enclose public lands with Yellowstone National Park in the center
(rectangle [this needs to be labeled]). (b) Distribution of primary
productivity across GYE as derived from the MODIS satellite
product. (c) Locations within GYE of native ungulate migration
routes (arrows), high bird species richness (red), and rural
residential home development (blue). Figure 4. Model depicting
subpopulations in a protected area being sources that are
vulnerable to loss to attractive sinks. Under natural conditions,
habitat quality is sufficiently high over the landscape such that
the entire population is a source (top). Human induced mortality
reduces reproduction in unprotected parts of the landscape creating
an attractive sink, and loss of individuals to this sink puts at
risk the source in the protected area (bottom). Figure 5. Effect of
location on estimates and 95% confidence intervals of annual
survival for female (F ) and male (M) study sample grizzly bears in
the Greater Yellowstone Ecosystem, 1983–2001. Locations were inside
Yellowstone National Park (InYNP), inside the Grizzly Bear Recovery
Zone (RZ) but outside YNP (OutYNP), and outside the RZ (OutRZ).
From Schwartz et al., 2007. Figure 6. Modeled sources (white) and
sinks (blue - due to low reproduction and high mortality; and
orange – due to low birth rates of grizzly bears in the Greater
Yellowstone Ecosystem based on a female survival cutoff of 0.9.
From Schwartz et al. 2010). Figure 7. Model depicting the protected
area as a source that contributes emigrants to surrounding areas
sustainable harvest creates an attractive sink.
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Table 1. Simulated net population growth of Yellow Warblers
across the Greater Yellowstone Ecosystem across land allocations
with and without the effect of land use. Modified from Hansen and
Rotella (2002).
-35 -28
804 984 National Park
6 41 2003
4251 National Forest
-85
309
2942 808 Private
Net Population change with homes
Net population change w/o homes
Current Population size
Area (ha) Land Type
= 1.06 = 0.98
